Rationalize test fixtures

[rod-glover]

Resolves #41

The main motivation for this PR is, as noted in #41, is to pave the way for a reasonable implementation of #32 (variable schema name), which is almost complete but foundering on test insanity.

There are now substantially fewer and simpler test fixtures, and they are much more consistent with each other.

Rationalizing the test fixtures led naturally to a hierarchy of test subdirectories, with test-specific fixtures isolated in deeper conftest.py files. This makes the relationship of the fixtures to each other and to test code much easier to understand, and the tests are now better organized and more navigable too.

Also removed a pile of cruft and orphaned code.

Notes to reviewers:

1. The tests proper have hardly changed at all. You can safely ignore all files of the form test_*.py, and assume (rightly) that they have simply been moved to a new location and wired up to the new, rationalized fixtures.
2. All of the important changes are in the various conftest.py files.

[rod-glover]

There is still some DRYing up of code that could be done:

• creation and removal of a set of views in a fixture is copy-pasted in several places
• addition of test database objects from pycds/data/crmp_subset_data.sqlis repeated

[rod-glover]

OK, that's it. As squeaky-clean as it's going to get.

[rod-glover]

This proved out by making the implementation of #32 very straightforward (in the testing realm). Since I need to move forward with this sequence of PRs, I am going to merge this one without review.